1. Field of the Invention
The present invention relates to a color cathode ray tube having an in-line type electron gun.
2. Description of the Related Art
A color cathode ray tube (hereinafter, referred to as a color CRT) employing an in-line type electron gun is used in TV sets, computer monitors, and the like. In such a color CRT, in order to make three electron beams emitted from the electron gun converged on a screen, the directions of the respective outermost electron beams are changed so as to approach the center electron beam in the electron gun, as disclosed in, for example, Japanese Patent Publication No. 1-29299. The convergence of the electron beams is described with reference to FIG. 1.
FIG. 1 is a cross-sectional view of an electron gun 12 provided inside a typical color CRT and a glass panel 7 provided in front of the color CRT. The electron gun 12 includes cathodes 1a, 1b, 1c for emitting electrons and electrodes 3 composed of first to fourth grid electrodes G1-G4 disposed in front of the cathodes 1a, 1b, 1c. Intensity and focusing of electron beams 2a, 2b, 2c emitted from the cathodes 1a, 1b, 1c are controlled by applying different potentials to each of the grid electrodes G1-G4.
Hereinafter, control of the electron beams 2a, 2b, 2c is further described in detail.
The electron beams 2a, 2b, 2c corresponding to red, green and blue are emitted from the cathodes 1a, 1b, 1c, respectively. The electron beams 2a, 2b, 2c travel through the first to fourth grid electrodes G1-G4. After traveling through apertures 5 which are formed on a shadow mask 4 having a color selecting function, the electron beams 2a, 2b, 2c reach a phosphor screen 6 provided on the inner surface of the glass panel 7 so that phosphor materials emit light. Although not shown in FIG. 1, a deflection yoke (see FIG. 3) is disposed between the electron gun 12 and the glass panel 7. The deflection yoke deflects the electron beams 2a, 2b, 2c so as to scan the phosphor screen 6.
Moreover, in order to obtain an image of high quality, it is necessary to obtain sufficient convergence of the electron beams 2a, 2b, 2c on the phosphor screen 6. For this purpose, the through-holes of the second to fourth grid electrodes G2, G3, G4 through which the electron beams 2a and 2c travel are made eccentric among the grid electrodes G2, G3, G4 so as to form the gradient of the electric field. Thus, the directions of the respective outermost electron beams 2a and 2c on each side are changed by the gradient of the electric field among the grid electrodes G2, G3, G4 so as to gradually approach the central electron beam 2b.
As described above, the electron beams 2a, 2b, 2c are controlled inside the color CRT. However, when the color CRT operates with an external magnetic field existing, the interaction between the electron beams and the external magnetic field necessarily occurs. As a result, convergence of the electron beams cannot be properly obtained, i.e., misconvergence occurs. Even if an artificial external magnetic field does not exist, the effects of geomagnetism are inevitable.
Normally, in order to avoid the misconvergence due to the external magnetic field as described above, a magnetic shield member is provided along the inner surface or the outer surface of the color CRT between the deflection yoke and the glass panel 7 so as to enclose the electron beams 2a, 2b, 2c.
By providing the magnetic shield member, the effects of an external magnetic field in the perpendicular direction to the electron beams 2a, 2b, 2c are controlled. Strictly speaking, the effects of the external magnetic field in the direction along the straight line connecting the center of the electron gun 12 and the center of the phosphor screen 6 (hereinafter, referred to as "axis direction of the electron gun") cannot be controlled. However, the effects of the external magnetic field in the axis direction of the electron gun are not conventionally regarded as problems to be immediately solved, since the effects are generally negligibly small.
Recently, however, there have been great demands for a color CRT having high performance properties, especially to improve the quality of a resultant image. As a result, it has become necessary to pay attention to the insignificant deterioration of the quality of image due to misconvergence caused by an external magnetic field in the axis direction of the electron gun as described above. Thus, solution of the above described problem is now in urgent demand.